Screening machines utilizing a plurality of frustum conical screens are well known. Early screening machines of this kind, for example German Auslegeschrift No. 12 32 005, incorporate frustro-screens which are stationary. The material to be seived is introduced substantially in the form of a suspension tangentially with a charging velocity that allows the suspension to follow a helical path on its decent along the internal screen surface. Also suggested were screening machines wherein a frustro-conical screen barrel was rotated. See, e.g. German Patent Specification No. 816,051. These prior screening machines, however, were not suitable for high throughputs of moist material.
In those cases where screening is difficult, such as large throughputs of moist material, special screening machines have been proposed which operate on the principle of high speed thin film (layer) screening. These machines are typically fitted with a movable elastic screening deck, a mechanically moved screening deck, or a special tapping device or the like.
For the achievement of high screening capacity, it is desirable that the screening deck provide vibratory motion over practically its entire surface. Dead zones or zones devoid of vibratory motion on screening deck surface is undesirable.
One such device for providing high accelerations on vibratory screens was disclosed in German Patent Specification No. 12 06 372. That device included at least two screen frame systems which moved relative to one another. The grate bars meshed in pairs and were joined by screening deck elements that movably linked to the bars. The deck elements bridged the space between the bars which were spaced sufficiently far apart so as to allow their relative movement in relation to each other. The relative movement of the grate bars occurred at right angles to their length so as to produce therebetween zones of continuously varying widths. The screen positioned between the bars would alternate or vary in tension as a function of the properties, design and construction of the selected elastically stretchable screening sheets. The disadvantage of this device is the need for two opposing drive systems to produce the relative motion between adjacent bars to generate the so-called "springboard effect" or "trampoline effect" of the screen.
In German Auslegeshrift No. 21 03 098 there is disclosed a vibratory screen having a stretched screening deck of elastically stretchable material. The screen is held only at its edges which are at right angles to the conveying direction. Attached to the deck are transverse members which are driven together with the deck.
When such machines as those described are designed for high feed rates and throughputs, they tend to become extremely heavy so that correspondingly high static and dynamic loadings occur in the region of the support structure. These loadings are transmitted to the buildings in which they are housed creating difficult structural problems. Moreover, when the screening machines are fitted with positively moved elastic screening decks or screening sheets, optimum operation, especially where continuous operation is required, is not attained because their elaborate mechanical construction is subject to frequent breakdowns.
An improvement was proposed in such devices in German Patent Specification No. 1 757 423 where a screening machine having a substantially horizontally held nonrotating circular screen is disclosed which has imparted to it a substantially circular horizontal vibratory motion as well as a vertical vibratory component by means of an eccentric drive.
Notwithstanding the numerous types of screen devices proposed or available, few are capable of high throughputs of moist material. Those which are, have complicated mechanical components. Accordingly, it is an object of the present invention to provide a screening machine which can be constructed for the treatment at high throughput rates of moist charge material. It is a further object to provide such a machine which is relatively uncomplicated and requires only one drive system to achieve rapid vibratory motions or high accellerations of a screening deck. It is yet a further object to provide a machine in which a mass of material undergoing screening is kept as small as possible through high vibratory energy.